Duomode microwave radiation system



April 18, 1961 s. M. KERBER El'AL DUOMODE MICROWAVE RADIATION SYSTEM Filed April 5, 1957 I III/II I l L- INVENTORS. STANLEY M. KERBER GRANT M. RANDALL BY DONALD 39ZEMKE AGENT beams of electromagnetic waves.

Patented Apr. 18, 1961 DUOMODE MICROWAVE RADIATION SYSTEM Stanley M. Kerber, Fullerton, and Grant M. Randall and Donald F. Zemke, Whittier, Calif., assignors to North American Aviation, Inc.

Filed Apr. 5, 1957, Ser. No. 651,105

4 Claims. (Cl. 343-779) This invention relates to microwave transmission, and more particularly to an improved feed device for transmitting microwave energy.

In antennas employing a parabolic reflector a feed is required to direct microwave energy on the surface of the reflector to produce optimum radiation patterns. The main function of the feed is to transmit and receive the microwave energy. In order to perform this function efliciently and accurately the feed must be designed to match the impedance between the wave guide and free space existing between the antenna and the target, to transmit microwave energy without undue loss of energy within the feed, and to provide an optimum effective 'area for propagating the energy.

Conventional antennas employ feeds and associated wave guides wherein only one mode of propagation is supported by the guide. Simultaneous illumination of a reflector antenna by a feed containing two. modes of propagation was considered impractical for the reason that only one mode could be successfully coupled from the feed to free space with proper impedance and other radar characteristics. Thus the energy from a second mode being transmitted through the wave guide was not utilized. In addition conventional horn rear feed radar antenna systems are too bulky and heavy with comp-licated mechanisms.

This invention contemplates a microwave radiation feed system for simultaneously transmitting and receiving electromagnetic energy in two modes of propagation. A feed system comprising two adjacent rectangular wave guides with a common wall between is used to transmit electromagnetic energy optimumly upon a suitable parabolic reflector. The two wave guides in the feed system are each designed to support the two modes of propagation required by theradar system and to provide a proper impedance match between the feed system and the free space from the antenna to the target. This arrangement of a dual wave guide system provides an efflcient and versatile antenna system of light weight.

It is therefore an object of this invention to provide an improved microwave radiator.

It is another object of this invention to provide a feed system for propagating electromagnetic energy in two transverse electric'modes within the feed.

It is still another object of this invention to provide a feed system for a parabolic radar reflector which properly matches the impedance of the feed system to that of the free space into which the radar beam is projected.

It is a. further object of the invention to provide a radar feed system which permits attainment of the optimum illumination of an associated antenna.

It is still a further object of this invention to provide a microwave radiator adapted to radiate sharp conical Other objects of this invention will become apparent from the following descriptions taken in connection with the accompanying drawings in which:

Fig. 1 is a plan view of the invention;

Fig. 2 is a sectional view of the invention taken at:

22 in Fig. 1;

Fig. 3 is a sectional view of the invention taken at 3--3 in Fig. 2;

Fig. 4 is a plan view of the invention; and

Fig. 5 is an elevation view of the invention.

A wave guide may transmit electromagnetic waves in a variety of modes, depending upon the relation between the geometry of the guide and the nature of the generated waves. This invention concerns itself primarily with a class of modes of transmission known as transverse electric. In transverse electric waves the electric field has components only normal to the direction of propagation guide axis and the electrical field is transverse to the axis. Transverse electric waves will be identified, for discussion purposes, by indicating by subscript the number of half-period variations in transverse field intensity along the short and long dimension of the wave guide cross-section, respectively. For instance, the dominant mode in a rectangular wave guide is the TE mode indicating that the wave is of the transverse electrical type, that there is no variation in the transverse field in the short dimensions, and that there is a single half-wave variation of transverse field along the long dimensions thereof. Similarly, the TE mode indicates there is no variation along the short-dimensioned wave guide cross-section and two half-wave variations of transverse field along the long dimensions.

In accordance with the device of this invention a feed system is employed to propagate two modes of electromagnetic energy. The feed system comprises two contiguous wave guides each capable of supporting the two modes (TE and T1302) of propagation necessary for an associated radar system which is not a part of this invention. The TE mode of propagation may be used, for example, by the associated radar to determine the range and azimuth error of a target, and the TE mode may be used to determine elevation error. The two modes are thus utilized by the radar to provide three dimensional information about a target. Since each wave guide will support, both modes of propagation at the same time, the range and azimuth error and elevation error may be obtained simultaneously.

The manner in which the TE and TE mode electromagnetic Waves reflected from the target and propagated through the device of this invention are utilized and separated is fully described in patent application Serial No. 216,145, entitled Duomode Monopulse Radar System, filed April 6, 1951, in the name of Robert M. Ashby. The apparatus described in the application just referred to cooperates with this invention to provide electrical signals of range, azimuth and elevation of a target.

Referring now to Fig. 1, a double wave guide 1, such as is shown in detail in patent application Serial No. 216,145, filed April 6, 1951, in the name of Robert M. Ashby for Duomode Monopulse Radar System, projects through parabolic reflector 2 and is connected to tapered colinear feed section 3, which in turn is connected to bend section wave guides 5 and 6 is shown in detail in Fig. 3, each of which effect a reversal of the direction of propagation and are connected to matching plate 7 having steps 8, 9, 10, 11, 12 and 13. Wave guides 5 and 6 are uniformly rectangular in cross-section throughout the straight and bent portions as shown in Figs..1 and 4. Rectangular wave guides sections 5 and'6 are bent as sharply as possible in order to keep over-all dimensions of the guides at a minimum. Matching plate 7 is symmetrically positioned with respect to parabolic reflector 2, such that the effective center of radiation of the output slots of steps 10 and 13 is located so as to provide a proper impedance match between the feed system and free space, so that the microwave energy propagated through the output slots of steps 10 and 13 may properly illuminate the surface of reflector 2, and in order that microwave energy reflected from a target and collected by the reflector will be efficiently focused upon the slot openings of steps 10 and 13, and thence transmitted back through wave guides 5 and 6 to the radar system.

Wave guide 3 is tapered gradually in order that no harp discontinuities, the effect of which would be to set up standing waves within the guides and thus produce wasted energy, occur. The presence of too high a standing wave within the wave guide system will result in detrimental effects upon the operation of the microwave generator including a loss of power. Wave guide 3 must be tapered to a narrow dimension as possible so that output slots of steps 10 and 13 may be placed at a space which will yield optimum reflector illumination. If the centers of these output slots of steps 10 and 13 are separated by more than approximately of the wave length propagated, side lobe radiation from the wave guide illuminates the reflector, the gain of the antenna is reduced, and the beam width is increased. On the other hand, if the spacing is less than /8 of the wave length, microwave energy will spill over the edges of the reflector, resulting in wave energy and reduced efficiency of operation. Wave guides 5 and 6 are bent as shown without sharp corners in order that the waves propagated within the guide can be directed toward the reflector without introdueing reflections of the energy within the guide system. Matching plate 7 having steps 8, 9, 10, 11, 12 and 13 operates to match the impedance between wave guides 5 and 6 and free space. In wave guide 6 steps 11 and 13 are matched in impedance by step 12 and in wave guide 5 steps 8 and 10 are matched in impedance by step 9. The width of steps 8, 9, 10, 11, 12 and 13 has been found to be optimum at approximately ,6 wave length, and the difference in height of the guide for the steps shown in Fig. 3 may be as great as 3 of a wave length.

The width of guides and 6 must be above a certain critical dimension in order that transverse electric waves having both one and two half-wave amplitude variations in electric field intensity across the wide dimension of the guide may be propagated therethrough. This critical dimension, known as the cut-off dimension, is equal to the free space wave length of the microwave energy being propagated. However, since the attenuation in a wave guide for a mode of transmission near cutoff is relatively high, the width is preferably made somewhat greater than this cutoff wave length. If these slots are of a length of to 12% greater than the wave length of the microwave energy being transmitted, satisfactory results will be obtained. The width of the guide must not exceed this value too greatly, however, or the radiation pattern in the plane of the septum between guides 5 and 6 may be too narrow, and side lobe radiation will illuminate the reflector.

The curved portions of wave guides 5 and 6 effect an I80 degree reversal of the direction of propagation of the waves. In order to do this the structure of the guides is bent substantially more than 180 degrees. Generally the bend radius of guides 5 and 6 must be small enough so that the guides do not project beyond the edges of plate 7. The dimensions of plate 7, in turn, are determined by a compromise between the requirement of. minimum aperture blocking and the function of the plate in helping to direct energy at the reflector in order to obtain an optimum sharp pencil beam. It has been found satisfactory to use a bend radius as small as 3 of the wave length for wave guides 5 and 6. The lengths of each step and the change of guide height of each step are interrelated in their effect upon the amplitude and phase of reflected energy from each of the discontinuities, and are adjusted to obtain a negligible net reflection in guides 5 and 6 throughout the band of frequencies propagated for transverse electric modes having both one and two half-wave variations in electric field intensity over the width of the guide. The band of frequencies propagated is narrow relative to the actual center frequency, being +6% to -10% of that frequency. This is a broad range in comparison to other feed devices, however, giving this feed the advantages of interchangeability of frequency determining radar components in the system with which it is used. The width of wave guides 5 and 6 in Fig. 3 depends upon the atmosphere and pressure therein as well as the peak power transmitted, but may safely be as small as wave length without danger of breakdown at atmospheric pressure when 250 kilowatt peak power is used.

In operation, energy in the TE mode is propagated by means known in the art including a microwave generator through wave guide section 1 and is split into left and right guides 5 and 6 which propagate the TE mode with equal intensity and phase. The energy is reversed in direction and illuminates reflector 2 which propagates the energy into free space in a lobe centered on the antenna axis. The reflected energy from a target is carried back through the feed system in the same manner as it was sent out. If the target is in such a position as to cause the reflected wave to be incident upon one side or the other of wave guides 5 and 6 before reaching the other side, a complex electromagnetic field in each wave of guides 5 and 6 is excited. This situation will occur when the target is, for example, off axis in azimuth. This field may be resolved into two TE mode components which differ in amplitude and phase and are separated by a radar system not a part of this invention. Similarly, for targets off axis in elevation, the combination mode may be resolved into a TE and a TE mode and separated by the radar system to provide the necessary three dimensional information. Wave guides 5 and 6, both capable of transmitting the TE or TE mode, propagate these modes back through the feed system to the radar system.

By proper combination of wave guide and associated component dimensions it is possible to transmit both the TE and TE mode electromagnetic energy accurately and efliciently successfully matching both modes to free space.

The accompanying table gives typical dimensions of a feed system constructed according to this invention in terms of the center wave lengths A Diameter of reflector 2 22.2A Focal length of reflector 2 9.14.. Distance from matching plate 7 to reflector 2 9%,, Width of output slots and-guides 1.1 14x Width of guides 5 and 6 0.0995) Length of steps 8 and 11 0.105) Length of steps 9 and 12 0.0987) Length of steps 10 and 13 0.0787) Width of guide for steps 8 and 11 0.1352) Width of guide for steps 9 and 12 0.167) Width of guide for steps 10 and 13 0.207) Distance of output slots of steps 10 and 13 from outside surfaces of guides 5 and 6 .0462).

' not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim:

1. Microwave radiation and receiving apparatus for transmitting and receiving electromagnetic energy in two modes of transmission comprising a parabolic reflector. a pair of contiguous wave guides projecting through said reflector at its axis of symmetry, said guides tapering in height dimension to a minimum in the region of the focal point of said parabolic reflector, a circularly bent 2. Microwave radiation and receiving apparatus for transmitting and receiving electromagnetic energy in two modes of transmission between a source and free space comprising a parabolic reflector having an aperture defining a principal direction of wave propagation, a pair of contiguous rectangular wave guides each greater than one wave length in width projecting through said reflector at its axis of symmetry, said guides tapering in width dimnesion to a minimum in the region of the focal point of said parabolic reflector, a conductive wall common to said wave guides, circularly bent rectangular wave guide connected to each said wave guide for directing said microwaves generally at said reflector, each said circularly bent wave guide having a first portion bent substantially more than 180 degrees and having a second portion bent a number of degrees in a reverse direction equal to the excess of the bend of said first portion over 180 degrees, a slot aperture at the end of each said bent guides for illuminating said reflector with said microwaves, and a plurality of enlarging rectangular stepped discontinuities in the height dimensions of said slot apertures joining said bent guides and said slot apertures for watching the impedance of said guides to the impedance of free space and for producing negligible reflections of microwave energy in said wave guides.

3. Microwave radiation and receiving apparatus for transmitting and receiving electromagnetic energy in two modes of transmission between a source and free space comprising a parabolic reflector having an aperture defining a principal direction of wave propagation, a pair of direction contiguous rectangular wave guides each greater than one wave length in width projecting through said retbctor at its axis of symmetry, said guides tapering in width dimension to a minimum in the region of the focal point of said parabolic reflector, the center spacing of said wave guides not less than 7% nor more than of the wave length of the mean propagated frequency in free space, a conductive wall common to said Wave guides, a circularly bentrectangular wave guide connected to each said wave guide for directing said microwaves generally at said reflector, each said circularly bent wave guide being bent substantially more than degrees for a portion thereof, a slot aperture at the end of each said bent guides for illuminating said reflector with said microwaves, and a plurality of enlarging rectangular stepped discontinuities in the height dimensions ,of said slot apertures joining said bent guides and said slot apertures for matching the impedance of said guides to the impedance of free space and for producing negligible reflections of microwave energy in said wave guides.

4. A microwave radiator for radiating and receiving electromagnetic waves propagated in two transverse electric types modes comprising a pair of parallel rectangular wave guides each greater than one wave length in width projecting through said reflector at the axis of symmetry thereof and each including a curved portion for eifecting 180 degrees reversal of the direction of propagation of said waves, said curved wave guide portion comprising a portion bent substantially more than 180 degrees and a reverse bend portion for directing waves to free space, and means for matching the impedance of said wave guides to the impedance of free space to thereby radiate and receive electromagnetic waves propagated through said guides in two modes.

References Cited in the file of this patent UNITED STATES PATENTS 2,283,935 King May 26, 1942 2,729,817 Cornbleet Jan. 3, 1956 2,784,403 Marsh Mar. 5, 1957 2,803,817 Marasco Aug. 20, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 2,980,911 I April 13, 1961 Stanley M. Kerber et a1.

It is hereby certified that error appears in :bhe above'numbered patent requiring correction and that the said Letters" Patent should read as corrected below.-

Column 5, line 34. for "watching" read matching Sigmed and sealed this 20th day of March 1962.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer I Commissioner of Patents 

